Image sensor and pixel arrary

ABSTRACT

A pixel array comprises at least one long exposure pixel, short exposure pixel, and a control circuit. The long exposure pixel comprises a first photodiode to generate charges, a first image signal generating module for generating a first image sensing signal; and a first transfer switch device for passing the charges to the first image signal generating module via a first transfer control signal. The control circuit sets the first transfer control signal to be a first predetermined control voltage when the long exposure pixel is in an long exposure phase, and then sets the first transfer control signal to be a second predetermined control voltage when the short exposure pixel is in a short exposure phase.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image sensor, and particularlyrelates to an image sensor with an anti-blooming mechanism.

2. Description of the Prior Art

A pixel of an image sensor has a photodiode to generate chargesaccording to light and a transfer switch coupled to the photodiode tocontrol the charges selectively output to a floating diffusion. In orderto well control the charge leakage, for example the transfer switchbeing an n-type transistor, a negative control voltage is provided to acontrol terminal of the transfer switch. Under exposure, the photo diodeconverts light into charges. If the light is too bright, however, alarge quantity of charges are generated and accumulated in thephotodiode, and leakage current may likely occur to affect theneighboring pixels. Such situation is called blooming.

Therefore, a good mechanism for avoiding the bloom issue is needed.

SUMMARY OF THE INVENTION

Therefore, one objective of the present invention is to provide an imagesensor that can avoid the bloom issue.

Another objective of the present invention is to provide a pixel arraythat can avoid the bloom issue.

One embodiment of the present invention discloses A pixel arraycomprises at least one long exposure pixel, short exposure pixel, and acontrol circuit. The long exposure pixel comprises a first photodiode togenerate charges, a first image signal generating module for generatinga first image sensing signal; and a first transfer switch device forpassing the charges to the first image signal generating module via afirst transfer control signal. The short exposure pixel comprises asecond photodiode to generate charges; a second image signal generatingmodule for generating a second image sensing signal; and a secondtransfer switch device for passing the charges to the second imagesignal generating module according to a second control signal. Thecontrol circuit sets the first transfer control signal to be a firstpredetermined control voltage when the long exposure pixel is in an longexposure phase, and then sets the first transfer control signal to be asecond predetermined control voltage when the short exposure pixel is ina short exposure phase.

The above-mentioned pixel array is not limited to be applied to theimage sensor but can be applied to other devices.

In view of above-mentioned embodiments, the control terminals for thelong exposure pixel and the short exposure pixel are asserted to ahigher level before the data in the pixels are read. Therefore the photodiode accumulates less charge and the leakage current is suppressed. Bythis way, the bloom issue can be reduced.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an image sensor according to anembodiment of the present invention.

FIG. 2 is an example for an exposure pattern comprising long exposurepixels and short exposure pixels.

FIG. 3 is a circuit diagram illustrating a long exposure pixel and ashort exposure pixel according to an embodiment of the presentinvention.

FIG. 4 is a timing diagram illustrating the reset operation for theimage sensor shown in FIG. 2.

FIG. 5 is a wave chart illustrating the reading operation for the imagesensor shown in FIG. 2.

DETAILED DESCRIPTION

FIG. 1 is a block diagram illustrating an image sensor 100 according toan embodiment of the present invention. As shown in FIG. 1, the imagesensor 100 comprises a pixel array 101, a control circuit 103, a readcircuit 105 and an analog to digital converter 107. The pixel array 101comprises a plurality of pixels PIX₁₁-PIX_(mn) to generate the imagesignals IS₁₁-IS_(mn). The read circuit 105 comprises a plurality of readunits S₁-S_(n), which are arranged to read image signals IS₁₁-IS_(mn)from the pixels PIX₁₁-PIX_(mn). The analog to digital converter 107generates a frame according the image signals IS₁₁-IS_(mn). The controlcircuit 103 controls the operations for each device in the image sensor100. That is, the control circuit 103 can control the reset time, theexposure time and the read operation for the pixels PIX₁₁-PIX_(mn).

The pixel array in an image sensor, in one embodiment, may have longexposure pixels and short exposure pixels, such that a high dynamicrange (HDR) image is generated. The bloom issue is seriously concernedif the long exposure pixel and the short exposure pixel are beside eachother. Therefore, a pixel array having long exposure pixels and shortexposure pixels is taken as example for the following embodiment.However, please note the scope of the present invention is not limitedto a pixel array having long exposure pixels and short exposure pixelsand is not limited to an image sensor with such pixel array.

As above-mentioned, the control circuit 103 can control the exposuretime for the pixels PIX₁₁-PIX_(mn). Therefore an exposure pattern can begenerated. FIG. 2 is an example for an exposure pattern comprising longexposure pixels and short exposure pixels. As shown in FIG. 2, rowsR₁-R₈ in the pixel array respectively have several short exposure pixelsS and long exposure pixels L. The short exposure pixels S are accessedvia accessing lines L_(a1)-L_(a8), and the long exposure pixels areaccessed via accessing lines L_(b1)-L_(b8). Please note the arrangementfor the long exposure pixels L and the short exposure pixels S, and thearrangements for the accessing lines L_(a1)-L_(a8)/L_(b1)-L_(b8) shownin FIG. 2 are only for example and do not mean to limit the scope of thepresent invention. Other arrangements should also fall in the scope ofthe present invention.

FIG. 3 is a circuit diagram illustrating a long exposure pixel and ashort exposure pixel according to an embodiment of the presentinvention. The long exposure pixel PIX_(L) (i.e. the long exposure pixelL in FIG. 2) comprises a first photodiode PD₁, a first image signalgenerating module SGM₁ and a first transfer switch device T_(X1). Thefirst photodiode PD₁ senses light to generate at least one charge. Thefirst image signal generating module SGM₁ generates a first imagesensing signal IS₁ (one of the image signals IS₁₁-IS_(mn)) according tothe charge generated by the first photodiode PD₁. The first transferswitch device T_(x1) passes the charge from the first photodiode PD₁ tothe first image signal generating module SGM₁ according to a firsttransfer control signal S_(x1).

The short exposure pixel PIX_(S) (i.e. the short exposure pixel S inFIG. 2) comprises a second photodiode PD₂, a second image signalgenerating module SGM₂ and a second transfer switch device T_(X2). Thesecond photodiode PD₂ senses light to generate at least one charge. Thesecond image signal generating module SGM₂ generates a second imagesensing signal IS₂ (one of the image signals IS₁₁-IS_(mn)) according tothe charge generated by the second photodiode PD₂. The second transferswitch device T_(x2) passes the charge from the second photodiode PD₂ tothe second image signal generating module SGM₂ according to a secondtransfer control signal S_(x2).

In one embodiment, the first image signal generating module SGM₁ and thesecond image signal generating module SGM₂ both have a 4 T pixelstructure. That is, the first image signal generating module SGM₁ has afloating diffusion FD₁, a first reset transistors T_(r1), a first sourcefollower T_(f1), and a first row selecting transistor T_(s1), the secondimage signal generating module SGM₂ has a floating diffusion FD₂, asecond reset transistors T_(r2), second a source follower T_(f2), and asecond row selecting transistor T_(s2). Persons skilled in the art caneasily understand the operation the first image signal generating moduleSGM₁ and the second image signal generating module SGM₂, thus are notdescribed in detail. However, please note the circuit structures for thefirst image signal generating module SGM₁ and the second image signalgenerating module SGM₂ in FIG. 3 are only for example and do not mean tolimit the scope of the present invention. For example, the selectingtransistors T_(s1) and T_(s2) can be omitted in other embodiments.

FIG. 4 is a timing diagram illustrating the operation for the imagesensor shown in FIG. 2. The image sensor 100 first enters the exposurereset period at time T₁. The reset signals RST₁ to RST₈ respectivelycontrol the reset transistor (ex. T_(r1), T_(r2) in FIG. 3) fordifferent rows. For more detail, the reset signal RST₁ controls thereset transistor in the pixels for the first row R₁ in FIG. 2, the resetsignal RST₂ controls the reset transistor in the pixels for the secondrow R₂ in FIG. 2, and so on. In this embodiment, the first row R₁ istaken for example, and the long exposure pixel PIX_(L) and the shortexposure pixel PIX_(S) are in the first row R₁ (but not limit). Thetransfer control signals, L₁, S₁, L₂, S₂ and etc. indicate the signalsfor respectively controlling the transfer switch devices for longexposure pixels and short exposure pixels of different rows. Forexample, the first transfer control signal L₁ indicates the signal forcontrolling the transfer switch devices for long exposure pixels in thefirst row R₁, and the second transfer control signal S₁ indicates thesignal for controlling the transfer switch devices for short exposurepixels in the first row R₁. Therefore, the first transfer control signalL₁ indicates the first transfer control signal S_(X1) in FIG. 3, and thesecond transfer control signal S₁ indicates the second transfer controlsignal S_(X2) in FIG. 3. The reset and exposure are in a row-by rowbasis and only the operation of the first row is described.

As shown in FIG. 4, in the exposure reset period, the control circuit103 first resets the long exposure pixels of the first row at a firstresetting time T₁ by asserting the reset signals RST₁ and the firsttransfer control signal L₁ to turn on the first row of the resettransistors and the transfer switch devices of long exposure pixels. Itshould be noted that the second transfer control signal S₁ may be or notasserted. After the long exposure pixel reset, the reset signals RST₁and the first transfer control signal L₁ are de-asserted to turn off thefirst row of the reset transistors and the transfer switch devices oflong exposure pixels, wherein the first transfer control signal L₁ isset to a first predetermined control voltage, which is −1V in thisembodiment, to fully turn off the transfer switch devices for longexposure pixels, and, in order to avoid charge leakage to theneighboring short exposure pixel, the first transfer control signal L₁is thereafter, in time T₄ in this embodiment, set as a secondpredetermined control voltage, which is 0V in this embodiment, tosubstantially turn off the transfer switch devices for long pixels butnot as tight as the first predetermined control voltage does. Then, atthe second resetting time point T₅, the control circuit 103 resets theshort exposure pixels of the first row by asserting the reset controlsignal RST₁ and the second transfer control signal S₁, such that theshort exposure pixels have shorter exposure time than the long exposurepixels.

In this embodiment, the second resetting time T₅ depends on the desiredexposure time of the short pixel. In the first row, the long exposurephase of the long exposure pixel is from time T_(1a) to time T_(re1),while the short exposure phase of the short exposure pixel is from timeT_(5a) to time T_(re1).

Please note that the above mentioned second predetermined controlvoltage (ex. 0v) is used to substantially turn off the transfer switchdevice, but not fully off as the first predetermined control voltage(ex. −1v), so as to provide another likely charge leakage path via thetransfer switch device to eliminate the charge leakage to the neighborpixels through other paths, such as the substrate. The first transfercontrol signal L₁, for the long exposure pixels, is first set at thefirst predetermined control voltage (−1V) at time T_(1a), and then setat the second predetermined control voltage (0V) just before the shortexposure phase of the short exposure pixels. Therefore, the firsttransfer control signal L₁ can be set at the second predeterminedcontrol voltage just before T_(5a) or earlier at T₄ in this example.

It is sufficient to use the second predetermined control voltage only inthe transfer control signal L₁ during or before the short exposure phaseof the short exposure pixel, while in this embodiment the secondpredetermined control voltage is used in both the transfer controlsignals L₁ and S₁ due to other design concern.

After exposure reset period, the image sensor 100 enters the read periodshown in FIG. 4 at time T_(re1). First, the reset transistors of a firstrow are turned on to reset the floating diffusions, such FD₁ and FD₂shown in FIG. 3, by asserting the reset signal RST₁ at time T_(re1).After floating diffusion reset, a first readout from the source follower(such as T_(f1) and T_(f2) in FIG. 3) is performed at time T_(re2) tooutput a first sample. Then the transistors of a first row are turned onfor the charges of the photodiodes to transfer to the floatingdiffusions, and a second readout from the source follower is performedto output a second sample at time T_(re3). The first read out and thesecond readout are used for charge couple sampling (CDS) readout. Theother rows are read same as the first row and details would not berepeated.

In the above-mentioned embodiments, the long exposure pixel and theshort exposure pixel are in the same first row R₁. However, theabove-mentioned reset operations can be applied to the long exposurepixel and the short exposure pixel in different rows. Also, the pixelarray according to the present invention is not limited to be providedin an image sensor. The pixel array can be provided in other devices.

In view of above-mentioned embodiments, an image sensor according to anembodiment of the present invention can be summarized as follows:

An image sensor comprises a control circuit, a pixel array and an analogto digital converter. The pixel array comprises at least one longexposure pixel and at least one short exposure pixel. The long exposurepixel comprises a first photodiode, for sensing light to generate atleast one charge, a first image signal generating module, for generatinga first image sensing signal according to the charge generated by thefirst photodiode; and a first transfer switch device, for passing thecharge from the first photodiode to the first image signal generatingmodule according to a first transfer control signal. The short exposurepixel comprises a second photodiode, for sensing light to generate atleast one charge; a second image signal generating module, forgenerating a second image sensing signal according to the chargegenerated by the second photodiode; and a second transfer switch device,for passing the charge from the second photodiode to the second imagesignal generating module according to a second transfer control signal.The control circuit sets the first transfer control signal to be a firstpredetermined control voltage to turn off the first transfer switchdevice when the long exposure pixel is in an long exposure phase, andthen the control circuit sets the first transfer control signal to be asecond predetermined control voltage slightly higher than the firstpredetermined control voltage to substantially turn off the firsttransfer switch device when the short exposure pixel is in a shortexposure phase.

The above-mentioned pixel array is not limited to be applied to theimage sensor but can be applied to other devices.

FIG. 5 is a wave chart illustrating the reading operation for the imagesensor shown in FIG. 2. As shown in FIG. 5, the reset signal RST forcontrolling the reset transistor is asserted first, therefore the resettransistors (ex. T_(r1), T_(r2)) are turned on first. After that, thecontrol signals S_(x) are asserted, therefore the transfer switchdevices (ex. T_(x1), T_(x2)) are turned on. Also, the read operation isperformed only when source follower transistors (ex. T_(f1), T_(f2)) areconductive, that is, the selecting signal SEL is high. However, pleasenote the image signal generating modules in the embodiments shown inFIG. 3 may have different structures, as above-mentioned. Therefore, thesignals shown in FIG. 5 may also have different relations if the imagesignal generating modules have different structures.

In view of above-mentioned embodiments, the control terminals for thelong exposure pixel or the short exposure pixel are asserted to a higherlevel before the data in the pixels are read. Therefore the photo diodeaccumulates less charge and the leakage current is suppressed. By thisway, the bloom issue can be reduced.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. An image sensor, comprising: a control circuit; apixel array, comprising: at least one long exposure pixel, comprising: afirst photodiode, for sensing light to generate at least one charge; afirst image signal generating module, for generating a first imagesensing signal according to the charge generated by the firstphotodiode; and a first transfer switch device, for passing the chargefrom the first photodiode to the first image signal generating moduleaccording to a first transfer control signal; at least one shortexposure pixel, comprising; a second photodiode, for sensing light togenerate at least one charge; a second image signal generating module,for generating a second image sensing signal according to the chargegenerated by the second photodiode; and a second transfer switch device,for passing the charge from the second photodiode to the second imagesignal generating module according to a second transfer control signal;wherein the control circuit sets the first transfer control signal to bea first predetermined control voltage to turn off the first transferswitch device when the long exposure pixel is in an long exposure phase,and then the control circuit sets the first transfer control signal tobe a second predetermined control voltage slightly higher than the firstpredetermined control voltage to substantially turn off the firsttransfer switch device when the short exposure pixel is in a shortexposure phase; an analog to digital converter, for generating a frameaccording to the first image sensing signal and the second image sensingsignal.
 2. The image sensor of claim 1, wherein the long exposure pixelis beside the short exposure pixel.
 3. The image sensor of claim 1,wherein the long exposure pixels and the short exposure pixels are allprovided in a row.
 4. The image sensor of claim 1, wherein the firstimage signal generating module comprises a first reset transistor andthe second image signal generating module comprises a second resettransistor; wherein the first reset transistor comprises a firstterminal coupled to a first voltage source and a second terminal coupledto the first transfer switch device; wherein the second reset transistorcomprises a first terminal coupled to the first voltage source and asecond terminal coupled to the second transfer switch device; whereinthe control circuit asserts the first transfer control signal to be thesecond predetermined control voltage level before the short exposurephase ends.
 5. The image sensor of claim 4, wherein the control circuitsets the first transfer control signal to be the first predeterminedcontrol voltage and the second control voltage before a readingoperation for the long exposure pixel.
 6. A pixel array, comprising: acontrol circuit; at least one long exposure pixel, comprising: a firstphotodiode, for sensing light to generate at least one charge; a firstimage signal generating module, for generating a first image sensingsignal according to the charge generated by the first photodiode; and afirst transfer switch device, for passing the charge from the firstphotodiode to the first image signal generating module according to afirst transfer control signal; at least one short exposure pixel,comprising; a second photodiode, for sensing light to generate at leastone charge; a second image signal generating module, for generating asecond image sensing signal according to the charge generated by thesecond photodiode; and a second transfer switch device, for passing thecharge from the second photodiode to the second image signal generatingmodule according to a second transfer control signal; wherein thecontrol circuit sets the first transfer control signal to be a firstpredetermined control voltage to turn off the first transfer switchdevice when the long exposure pixel is in an long exposure phase, andthen the control circuit sets the first transfer control signal to be asecond predetermined control voltage slightly higher than the firstpredetermined control voltage to substantially turn off the firsttransfer switch device when the short exposure pixel is in a shortexposure phase.
 7. The pixel array of claim 6, wherein the long exposurepixel is beside the short exposure pixel.
 8. The pixel array of claim 6,wherein the long exposure pixels and the short exposure pixels are allprovided in a row.
 9. The pixel array of claim 6, wherein the firstimage signal generating module comprises a first reset transistor andthe second image signal generating module comprises a second resettransistor; wherein the first reset transistor comprises a firstterminal coupled to a first voltage source and a second terminal coupledto the first transfer switch device; wherein the second reset transistorcomprises a first terminal coupled to the first voltage source and asecond terminal coupled to the second transfer switch device; whereinthe control circuit asserts the first transfer control signal to be thesecond predetermined control voltage level before the short exposurephase ends.
 10. The pixel array of claim 8, wherein the control circuitsets the first transfer control signal to be the first predeterminedcontrol voltage and the second control voltage before a readingoperation for the long exposure pixel.